The present invention relates to methods and apparatus for detecting multiple types of defects in an image. More specifically, the present invention relates to methods and apparatus for detecting defects having different levels of contrast on an image of a substrate. Typical substrates include liquid crystal displays (LCDs), plasma displays, semiconductor wafers, textiles, lumber, and the like.
The use of a flat panel liquid crystal displays (FPLCD), plasma displays, and other types of displays continues to grow rapidly. Consumer items such as portable televisions, video recorders, notebook computers, handheld computers, personal digital assistants (PDAs), engineering work-stations, high-definition televisions (HDTV), watches, and the like use such displays. Based upon the continued demand for such displays, industry has made massive capital investments in creating state-of-art manufacturing lines.
Despite such capital outlay, however, the industry still relies primarily upon human test operators to perform the final test and inspection of such substrates. The test operator performs a variety of visual inspections of each display for defects and accepts or rejects the display based upon the operator's perceptions. Such inspection includes, non-exclusively, pixel-based defects, such as pixels that are inoperative, and area-based defects, such as areas on the display where pixels are brighter or darker than surrounding pixels. The quality and completeness of the inspection is dependent on the individual test operator, who has been trained using limited samples of displays that are characterized as passing or failing. Accordingly, the inspection results are highly subjective, prone to error, and cannot be used consistently and efficiently to monitor, control, and improve the quality of the various manufacturing processes. In addition, the subjective testing criteria results in a lack of industry wide quality standards.
Effective process monitoring and control of display production cycles is possible only through quantitative inspection methods by way of automatic inspection machines. An example of one of the first automatic inspection machines was conceived by Photon Dynamics, Inc. (PDI) in 1992, assignee of the present application. FIG. 1 shows its roles during the final testing stages of a substrate such as the FPLCD manufacturing processes 10. The FPLCD undergoes steps from cell completion 20, first inspection via flat panel inspection system 30, module assembly 40, second inspection via flat panel inspection system 50, shipment 60, and incoming inspection via flat panel inspection system 70. A communication network 80 provides an interface between each of the inspections at the flat panel inspection system and a process control work-station 90.
Mura Defect
"Mura" defects are area-based contrast-type defects, where a group of pixels is brighter or darker than surrounding pixels, when they should be uniform. Mura defects are also known as "Alluk" defects. Generically, such contrast-type defects are termed "blobs" until they are classified as Mura defects. There are many stages in the manufacturing process where problems may result in Mura defects on the display. In the case of a FPLCD, such problems include mis-registration of a source or a gate line, contaminants between layers in the display, non-uniform liquid crystal layer, and others, as will be described below.
Different types of manufacturing problems typically result in Mura defects having different characteristic shapes. In the case of a FPLCD, for example, a source line registration problem causes vertical streaks on an display. Further, different types of manufacturing problems typically result in Mura defects having different levels of contrast. In the example of a FPLCD, a gate line registration problem causes high-contrast horizontal streaks on an display, and a non-uniform liquid crystal layer causes a low-contrast straw-type pattern. Further description of typical Mura defect characteristics and associated manufacturing problems can be found in co-pending application Ser. No. 08/651,417, filed May 22, 1996, now U.S. Pat. No. 5,917,935, assigned to the present assignee. Application Ser. No. 08/651,417, now U.S. Pat. No. 5,917,935 is herein by incorporated by reference for all purposes.
Because certain manufacturing problems cause certain types of Mura defects, identification and, elimination of such manufacturing problems often leads to the reduction of Mura defects during subsequent processing runs. However, it is often difficult to recognize low-contrast Mura defects in images when higher-contrast defects are also present. As a result, false-positive or false-negative identification of Mura defects often results in misidentification of sources or causes of the defects.
What is needed are improved techniques and apparatus for detecting Mura defects.